Towards understanding inter-strain attachment variations of Escherichia coli during transport in saturated quartz sand - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 27

3.1 Introduction
Being an important member of the normal intestinal micro-flora of humans and other mammals,
Escherichia coli has been widely used as an indicator micro-organism of fecal pollution
(Medema et al., 2003). But E. coli is more than just a harmless intestinal inhabitant; it can also be
a highly versatile, and frequently deadly, pathogen (Kaper et al., 2004). The most notorious E.
coli serotype is O157:H7 (Stenutz et al., 2006), which has been the cause of several large
outbreaks of disease in North America, Europe and Japan, related to drinking water (Hrudley et
al., 2003) and food (Grimm et al., 1995; Kaper, 1998; Ozeki et al., 2003; Ezawa et al., 2004).
This E. coli O157:H7 is a so called entero-hemorrhagic E. coli (EHEC), causing bloody diarrhea
(haemorrhagic colitis), non-bloody diarrhea and hemolytic uremic syndrome (Boyce et al.,
1995). Upon transport in aquifers, a variety of processes can have an impact on the interactions
of the traveling organism with aquifer material, resulting in different apparent travel velocities
and concentration changes along a flow line. One of the more important processes is the
interaction of the surface of the organism with the aquifer grain surface (Foppen and Schijven,
2006).
The E. coli surface contains several different structures, especially lipopolysaccharides,
autotransporter proteins, flagella, fimbriae, adhesins, curli, and porins (Van Houdt and Michiels,
2005). Lipopolysaccharides (LPS), also known as endotoxins, are anchored in the outer
membrane of E. coli . They consist of a common lipid A, a core region, and an O-antigen
polysaccharide, which is specific for each serogroup. LPS occupies 75% of the surface of the
bacterium, and E. coli is estimated to have 10 6 molecules per cell (Caroff and Karibian, 2003).
More than 180 different O-serotypes have been described, but the exact saccharide composition
of more than 50% of all known E. coli serotypes is still unknown (Stenutz et al., 2006).
Autotransporter proteins are secretory proteins, consisting of various units, causing large
polyproteins with an aminoterminal domain extending from the cell surface into the environment
(Henderson et al., 1998). Adhesive phenotypes have been attributed to a subfamily of E. coli
autotransporters, including Ag43, AIDA and TibA. Antigen 43 (Ag43) is a prominent surface
protein of E. coli (around 50000 copies per cell). This autotransporter protein is a self-
recognizing adhesin, which contains both receptor recognition and receptor target, and protrudes
approximately 10 nm beyond the outer membrane (Van Houdt and Michiels, 2005). Flagella are
made up of the protein flagellin.
The role of flagellar filaments, motility and chemotaxis in biofilm formation has been well
established (Van Houdt and Michiels, 2005). Also, fimbriae have been associated with host
tissue adhesion of important pathogenic E. coli strains. An overview of the most important
fimbriae and adhesins, mostly related to virulence factors of E. coli is given in Table 3.1 . The
most common adhesins found in both commensal and pathogenic E. coli isolates are Type 1
fimbriae, which are 7-nm wide, approximately 1-m long rod-shaped adhesive surface organelles
(Van Houdt and Michiels, 2005). A type I fimbriated cell can have up to 500 fimbriae, consisting
of up to 5 million proteins, representing about 8% of the total cellular protein (Schembri and
Klemm, 2001). Their importance in biofilm formation has been well studied (Van Houdt and
Michiels, 2005). E. coli strains often produce other fimbriae ( Table 3.1 ) which are classified on
the basis of their adhesive, antigenic or physical properties or on the basis of similarities in the